Thyroid hormones, which include thyroxine (T.sub.4) and triidothyronine (T.sub.3) are involved in regulating organic metabolism and energy balance, growth and development, and the activity of the nervous system of an individual. Thyroid hormones stimulate carbohydrate and lipid catabolism in most cells of the body and increase the rate of protein synthesis. Since their overall effect is to stimulate catabolism, thyroid hormones increase the basal metabolic rate. As a result of catabolism, heat is give off, raising body temperature, a phenomenon termed calorigenic effect. Furthermore, thyroid hormones regulate tissue growth, in particular in children. Thyroid hormones are also involved in increasing the reactivity of the nervous system, thereby resulting in increased blood flow, increased and more forceful heartbeats, increased blood pressure, increased motility of the gatrointestinal tract and overall increased nervousness.
Abnormal levels of thyroid hormones or abnormal processing of thyroid hormones by cells can result in various disorders and conditions. For example, deficiency of thyroid hormones during fetal development can result in fewer and smaller neurons, defective myelination of axons, and mental retardation. During the early years of life, deficiency of thyroid hormones result in small stature and poor development of certain organs, such as the brain and reproductive structures, which can result in cretinism, characterized by dwarfism and mental retardation.
In the adult, hypothyroidism can produce myxedema, characterized by swelling of the facial tissues. A person having myxedema has a slow heart rate, low body temperature, sensitivity to cold, hypersensitivity to certain drugs (e.g., narcotics, barbiturates, and anesthetics), dry hair and skin, muscular weakness, general lethargy, and a tendency to easily gain weight. Due to the slowing of the heart rate, a person suffering from myxedema may overwork the heart muscles, causing the heart to enlarge. A person having myxedema may also lack mental alertness, due to dulling of nerve reactivity. The symptoms of myxedema can be alleviated by the administration of thyroid hormones.
Other diseases characterized by hypothyroidism include autoimmune diseases, such as atrophic (idiopathic) and Hashimoto's thyroiditis.
An excess of thyroid hormones, which can result, e.g., from hypersecretion of thyroid hormones or from an abnormality in the response of cells to thyroid hormone may also cause various disorders, generally termed thyrotoxicosis. The most common cause of thyrotoxicosis (excessive amounts of thyroid hormones) is Graves's disease, also called exophthalmic goiter. This disease is characterized by an enlarged thyroid, a goiter, which may be two or three times its normal size. The disease is also characterized by edema behind the eye, which causes the eye to protrude (exophthalmos). Other characteristic symptoms of this disease include an abnormally high metabolic rate, which can produce a range of effects including an increased pulse, high body temperature, heat intolerance, moist, flushed skin, loss of body weight. A person suffering from a disease characterized by excessive thyroid hormone levels has an increased response of the nervous system, causing the person to become irritable. This person may exhibit tremors of the extended fingers. Such diseases are treated classically by administration to the subject of antithyroid drugs that suppress the production of thyroid hormones, by treatment with radioactive iodine that selectively destroys thyroid cells, or by surgical removal of at least a portion of the thyroid gland.
An excessive level of thyroid hormone may also be caused by ingestion of excess iodine or lower than average iodine intake, or alternatively it may be caused genetically, and results in a condition termed simple goiter. Transient thyrotoxicosis is also frequently associated with thyroiditis. Yet other conditions resulting in thyrotoxicosis include toxic multinodular goiter, toxic adenoma, factitious thyrotoxicosis, thyrotoxicosis due to excess thyroid stimulating hormone (TSH), toxic thyroid carcinoma, toxic struma ovarii, and familial dysalbuminemic hyperthyroxinemia (FDH).
Thyroid hormones exert their wide ranging biological effects by interacting with thyroid hormone receptors which are present on a wide variety of cells. Thyroid hormone receptors (TRs) include TR-alpha (TR-.alpha.), TR-betal (TR-.beta.1), and TR-beta2 (TR-.beta.2), which are members of the steroid/thyroid superfamily of receptors, belong to the class of nuclear hormone receptors or nuclear receptors. This superfamily of protein includes, but is not limited to, glucocorticoid receptors (GR), mineralocorticoid receptors, progesterone receptors, estrogen receptors, the estrogen-related receptors, vitamin D3 receptors, retinoic acid receptors (RAR), retinoic X receptors (RXR), aldosterone receptors, and androgen receptors, which share structural homologies. Nuclear receptors are intracellular receptors which mediate the effects of steroid and thyroid hormones, as well as the metabolites of vitamin A (retinoic acid) and other hormones. Upon hormone binding, some receptors are translocated from the cytoplasm to the nucleus where they control the transcriptional expression of certain hormone-responsive genes. This involves the binding of the receptors, often in homo- or heterodimeric form, to specific sequences in the target gene promoter.
Sequence comparisons (Krust et al., EMBO J. 5: 891, 1986) and structure-function analyses (Giguere et al., Cell 46: 645, 1986; Kumar et al., Cell 51: 941, 1987; Kumar and Chambon, Cell 55: 145, 1988; and Green and Chambon, Nature 325: 75, 1987) have shown that the receptors are composed of a series of conserved domains. The most highly conserved domain is the DNA binding domain located in region C (Krust et al., supra; Green and Chambon supra; and Evans and Hollenberg, Cell 52: 1, 1988) containing a 66-68 amino acid core composed of two zinc fingers (Schwabe et al., Nature 348: 458, 1990; Hard et al., Science 249: 157, 1990; and Luisi et al., Nature 352: 497, 1991) which is essential for recognition of regulatory elements (REs). Three amino acids adjacent to the N-terminal zinc finger of the DNA binding domain, known as the P-box, are critical for DNA sequence recognition (Mader et al., Nature 338: 271, 1989; Umesomo and Evans, Cell 57: 1139, 1989; and Danielson et al., 57 Cell 1131, 1989). The ligand binding domain, located C-terminal to the DNA binding domain in region E, is less well conserved among the receptors and contains a ligand-inducible transcriptional activation function (Green and Chambon, supra; Evans, supra; Wahli and Martinez, supra; Gronemeyer, supra; Giguere et al., Cell 46: 645, 1986; and Kumar et al., Cell 51: 941, 1987). Transcriptional activating domains have also been identified in the poorly conserved N-terminal A/B regions of the glucocorticoid and estrogen receptors (Giguere et al., supra; Kumar et al., supra; and Tora et al., Cell 59: 447,1989). Nuclear receptors and nucleic acids encoding such are further disclosed, e.g., U.S. Pat. No. 5602009, U.S. Pat. No. 5,552,271, U.S. Pat. No. 5,614,620 (androgen receptor), U.S. Pat. No. 5,606,021 (mineralocorticoid receptors), and U.S. Pat. No. 5,597,705 (thyroid hormone receptors). Nuclear hormone receptor binding elements are also further described in U.S. Pat. No. 5,597,693.
Furthermore, nuclear hormone receptors can have mutually antagonistic activities, such as transcriptional interference (Xhang et al. (1996) J. Biol. Chem. 271:14825). It has been proposed that this interference is due to the interaction of the activation domains of nuclear receptors with a common but limiting target protein in their signalling pathways. For example, several members of the steroid/thyroid superfamily have been shown to interact with basal transcription initiation factors, e.g., TFIIB, or TATA binding protein (TBP) or proteins from the TATA box-binding protein complex TFIID (Tong et al. (1995) J. Biol. Chem. 270:10601; Salovsky et al. (1995) Mol. Cell. Biol. 15:1554; and Jacq, et al. (1994) Cell 79:107). TRs have been shown to interact, e.g., with the related receptors for 9-cis-retinoic acid (9-cis-RA) and RXR. TR/RXR heterodimers have stronger DNA binding affinity relative to either receptor alone (Yu et al. (1991) Cell 67:1251). The yeast two-hybrid system has also been used to isolate cDNA encoding proteins that interact with the thyroid hormone receptor. (Lee et al. (1991) Endocrinology 9: 243-254; PCT patent application having publication No. WO 94/10338).